Conventionally, a vane anemometer has an impeller mounted on a shaft. Flow of air through the impeller produces rotation of the shaft; the speed of rotation of the shaft is indicative of the air speed. Rotation of the shaft is then measured by a known mechanical or electrical means. In many applications, vane anemometers are required to be highly accurate. If significant frictional forces are present, such as between the shaft and its bearings, some of the work produced by the air flow will be used to overcome this frictional force. Work used to overcome frictional forces will not be converted into shaft rotation. The result being that the measured speed will be less than the true air speed. Since frictional forces may be more pronounced at low speeds, the usefulness of an anemometer at low air speeds may be limited if the effects of friction are not minimized.
Typically, the effects of frictional forces are minimized by mounting the rotatable shaft in precision bearings. Since precision bearings minimize friction, the speed of rotation of the impeller shaft more closely approximates the true air speed. Frictional forces may be reduced employing sapphire "V" bearings in conjunction with hardened steel shafts having small spherically shaped tips at either of its axial ends. When the proper axial clearance is provided between the axial ends of the shaft and the bearings, the spherical tips roll around the V of the jeweled bearing and thereby reduce friction between these components. As will be appreciated by those skilled in the art, if the axial clearance is not set properly, the effects of friction will not be minimized because the shaft tips will contact the respective bearings excessively. Examples of vane anemometers using such bearings are the "TurboMeter" manufactured by Davis Instruments of Hayward, Calif., and the "Sky Watch Fun," available from Flytec USA, of Miami, Fla. or Speedtech Instruments of Great Falls, Va.
Although jewel bearings reduce friction, they have several disadvantages. For instance, one disadvantage of sapphire bearing assemblies is the tendency for the tip of the impeller shaft to suffer wear in response to the forces generated by the various loads imposed upon it. At higher wind speeds, wear can be significant and can lead to a significant degradation in accuracy. A further disadvantage of jewel bearing assemblies is vulnerability to damage caused by mechanical shock. For this reason, some companies provide such bearings with cushioned supports in order to limit shock loads. Bearings of this type are available from Bird Precision, of Waltham, Mass.
Yet another disadvantage of jewel bearings is the requirement to provide a precise axial clearance between the ends of the shaft and the bottom of the V jewel, and the necessity to provide a means to set and hold this clearance. Typically, jewel bearings are adjustable and the axial clearance is set by a skilled operator. For instance, the bearings may be threaded to the anemometer, so that, the position of the bearings relative to the shaft can be either retracted or advanced as needed. Adjustable jewels of this type are also available from Bird Precision.
One more disadvantage of jewel bearings is their susceptibly to contamination, which can also give rise to a loss of accuracy. For example, if a contaminant enters the area between the shaft end and the bearing, it will interfere with rotation of the shaft and damage the bearing. The disadvantages listed above are not limited to anemometers having jewel bearings. Ball or needle bearings are also subject to contamination, wear and damage.
If a jewel bearing or other type of bearing should fail due to either wear, mechanical shock, interference by contaminants or other causes, it will have to be either repaired or replaced. In most instances, these bearings are housed within the anemometer and are not easily accessible. Consequently, if the bearings should fail, typically the entire anemometer must be replaced. This can be expensive. Furthermore, even if the bearings are accessible, they typically cannot be replaced by the average user because replacing them involves setting the proper axial clearance as described above and requires particular skill.
The advantages of a durable vane anemometer and the disadvantage of bearings employed in anemometers are well known. The need for an anemometer that can be relatively easily repaired has also been recognized. For example, in U.S. Pat. No. 3,823,611 issued to Rudow, the advantage of an anemometer that is economically assembled and easily serviced is noted. Furthermore, in U.S. Pat. No. 4,078,426 issued to Cassini, the patentee describes a vane anemometer intended to address the problem of bearing wear and the necessity to replace the bearing one or more times during the desired useful life of the instrument.
Accordingly, it is desirable for a vane anemometer to have a bearing assembly that can easily be replaced. Furthermore, a vane anemometer that can be constructed without requiring a skilled operator to set the proper axial clearance between the bearings and the shaft is needed. An improved vane anemometer that mechanically insulates the bearings from shock loads or dynamic forces is also needed.
Devices have also been developed for determining the wind chill of the ambient air. As is known, the wind chill is a function of the wind speed and the temperature of the ambient air. Therefore, in order to determine the wind chill an instrument must detect both of these parameters.
Portable instruments, such as a portable vane anemometer, typically have a temperature sensing probe for measuring the temperature of the ambient air. One difficulty that in encountered is designing portable vane anemometers is that the temperature sensing probe is typically relatively fragile, and susceptible to damage from either static or dynamic loads. In order to protect the temperature sensing probe, prior art instruments have disposed the temperature sensing probe within the casing of the instrument. Such a device is illustrated in U.S. Pat. No. 5,008,775 (Schindler et al.) and is also exemplified in the commercially available product known as the "SILVA WINDWATCH.RTM." which is believed to distributed within the United States by Nexus Marine, Inc.
Although disposing the temperature sensing probe within the casing of the instrument may function to protect the probe from various loading conditions, it presents other drawbacks as well. For example, portable vane anemometers are typically transported or stowed in a person's pocket, hand or other locations. Therefore, while being transported or stowed, portable vane anemometers are not at equilibrium with the ambient temperature, but rather, the temperature of the place where they are being transported or stowed, such as a person's pocket. Depending on the climate, the difference in temperature between the ambient and the storage or transporting location may be significant, and as much as 60.degree. F. Because of this difference in temperature, when a portable instrument is removed from its storage location to the ambient, it may take a considerable amount of time for the temperature probe to reach equilibrium with the ambient. Thus, it may take a relatively long period of time to obtain an accurate temperature measurement and thus, an accurate wind chill measurement. In light of this, there is a need for a portable hand-held vane anemometer that can determine wind chill relatively quickly when moving from one thermal mass to another. It is also desired to provide a portable hand-held vane anemometer that accomplishes this while providing protection for the relatively fragile temperature sensor.
It is further desired to provide a vane anemometer that can determine a wide array of environmental parameters in addition to wind speed, air temperature and wind chill. This invention also includes a portable vane anemometer that can measure a variety of environmental parameters, such as wind chill, wind speed, humidity and heat index. Similar to determining temperature and wind chill, determining relative humidity generally involves sensors that are temperature dependent and relatively fragile. Therefore, in order to determine relative humidity is also important that a humidity sensor of a hand-held vane anemometer is thermally isolated from the anemometer casing and protected from mechanical shock. A portable vane anemometer of this type is included within this invention.